As steel pipe for line pipe used for trunk lines of pipelines important as methods for transporting crude oil, natural gas, and other materials over long distances, high strength, high toughness steel pipe for line pipe has been proposed (for example, Patent Document 1). Up to now, steel pipe for line pipe of the American Petroleum Institute (API) standard X70 (tensile strength 564 MPa or higher) or higher and up to X80 (tensile strength 620 MPa or higher) has been put into practical use, but for the purpose of increasing the efficiency of transport of crude oil and natural gas, studies have been made on increasing the internal pressure of pipelines. Along with this, X70 or higher or further X80 or higher high strength steel pipe for line pipe having a tensile strength of 600 MPa or more is being required to be made further higher in strength and greater in thickness.
Regarding higher strength, for example, if using X120 class line pipe having 900 MPa or higher tensile strength, it is possible to raise the internal pressure, that is, the pressure of the crude oil or natural gas, to about double that of X65 class line pipe, so it becomes possible to transport about double the amount of crude oil or natural gas. Further, if raising the strength of the line pipe and raising the internal pressure resistance strength, compared with increasing the thickness, it becomes possible to slash the material costs, transport costs, and on-site welding costs and possible to greatly raise the pipeline laying costs.
Further, a pipeline has to be superior in low temperature toughness since it is often laid in cold regions. Further, at the time of installation, the ends of the line pipes are joined together, so superior on-site weldability is also demanded. X120 class steel pipe for line pipe satisfying such demands and higher in strength than even the steel pipe for line pipe proposed in Japanese Patent Publication (A) No. 62-4826 is being proposed (for example, Japanese Patent Publication (A) No. 2004-52104). This is high strength steel pipe for line pipe of a microstructure of the base material mainly comprised of a mixed structure of bainite and martensite. Further, for increased thickness, methods of production of thick-gauge steel plate with a metal structure made fine bainite by controlled rolling and controlled cooling and having a good strength and toughness is being proposed (for example, Japanese Patent Publication (A) No. 2000-256777, Japanese Patent Publication (A) No. 2004-76101, and Japanese Patent Publication (A) No. 2004-143509).
Steel pipe for line pipe is produced by forming thick-gauge steel plate into a pipe shape by the UO process, making the ends abut against each other, and seam welding them. When toughness and productivity are demanded such as in high strength steel pipe for line pipe, the seam welding is preferably submerged arc welding from the inner surface and outer surface. When welding a steel material a plurality of times in this way, there is the problem that the weld heat affected zone (called the “HAZ”) coarsened by the heat input in the preceding welding is reheated by the heat input of the subsequent heat input and therefore the toughness drops.
As technology for improving the low temperature toughness of the HAZ of high strength steel pipe for line pipe, the method of utilizing intragranular transformation to make the structure of the HAZ finer has been proposed (for example, Japanese Patent Publication (A) No. 8-325635, Japanese Patent Publication (A) No. 2001-355039, and Japanese Patent Publication (A) No. 2003-138340). The method proposed in Japanese Patent Publication (A) No. 8-325635 causes the formation of acicular ferrite as nuclei for oxides. The method proposed in Japanese Patent Publication (A) No. 2001-355039 and Japanese Patent Publication (A) No. 2003-138340 forms intragranular bainite using composite inclusions of oxides and sulfides as nuclei.
Most such conventional high strength steel pipes for line pipe increase the hardenability, increase the Mo effective for raising the strength, and obtain a mainly bainite metal structure to improve the toughness, but recently a reduction in the content of the expensive element Mo is being sought. However, if reducing the Mo, the hardenability easily falls and intragranular bainite becomes harder to obtain, so securing low temperature toughness of the HAZ has been difficult. Further, conventional high strength line pipe has a thickness of at most less than 25 mm. Thick line pipe of 25 mm or more or 30 mm or more is being demanded.